观察者模式(Observer Pattern)

1.概念：
在对象之间定义一对多的依赖关系，这样一来，当一个对象改变状态，依赖它的对象会接到通知，并自动更新。
2.原则:

• 封装变化：找出程序中会变化的地方，然后将其和固定不变的方面分离。在观察者模式中会改变的是主题的状态，以及观察者的数目和类型。用这个模式你可以改变依赖主题状态的对象，却不用改变主题，这就是提前规划。
• 针对接口编程，不针对实现编程：主题和观察者都使用了接口。观察者利用主题的接口进行注册，而主题利用观察者的接口通知观察者；
• 多组合，少继承：观察者模式将许多观察者“组合”进主题中。对象中的这种关系不是继承产生的，而是在运行时利用组合的方式产生的。
• 为交互对象之间的松耦合设计而努力

3.JDK的支持
java.util.Observer 接口
java.util.Observerable 类

package java.util;
/**
 *  观察者
 */
public interface Observer {
    void update(Observable o, Object arg);
}

package java.util;
/**
 * 主题
 */
public class Observable {
    private boolean changed = false;
    private Vector obs;

    public Observable() {
        obs = new Vector<>();
    }

    /**
     *添加观察者
     */
    public synchronized void addObserver(Observer o) {
        if (o == null)
            throw new NullPointerException();
        if (!obs.contains(o)) {
            obs.addElement(o);
        }
    }

    /**
     * Deletes an observer from the set of observers of this object.
     */
    public synchronized void deleteObserver(Observer o) {
        obs.removeElement(o);
    }

    public void notifyObservers() {
        notifyObservers(null);
    }

    public void notifyObservers(Object arg) {
        /*
         * a temporary array buffer, used as a snapshot of the state of
         * current Observers.
         */
        Object[] arrLocal;

        synchronized (this) {
            if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }

    /**
     * Clears the observer list so that this object no longer has any observers.
     */
    public synchronized void deleteObservers() {
        obs.removeAllElements();
    }

    /**
     * Marks this Observable object as having been changed; the
     * hasChanged method will now return true.
     */
    protected synchronized void setChanged() {
        changed = true;
    }

    /**
     * Indicates that this object has no longer changed, or that it has
     * already notified all of its observers of its most recent change,
     * so that the hasChanged method will now return false.
     * This method is called automatically by the
     * notifyObservers methods.
     *
     * @see     java.util.Observable#notifyObservers()
     * @see     java.util.Observable#notifyObservers(java.lang.Object)
     */
    protected synchronized void clearChanged() {
        changed = false;
    }

    /**
     * Tests if this object has changed.
     *
     * @return  true if and only if the setChanged
     *          method has been called more recently than the
     *          clearChanged method on this object;
     *          false otherwise.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#setChanged()
     */
    public synchronized boolean hasChanged() {
        return changed;
    }

    /**
     * Returns the number of observers of this Observable object.
     *
     * @return  the number of observers of this object.
     */
    public synchronized int countObservers() {
        return obs.size();
    }
}